The use of whitening agents, or brighteners, in laundry applications to whiten fabrics has been widespread since the mid-1970s. Since then, much research has been carried out into the properties of such whitening agents, and many different compositions including such agents have been described in the literature. However, most applications of whitening agents have been for the purpose of fabric whitening, rather than whitening of the detergent composition itself.
WO 94/05761 discloses a process for preparing a high density granular detergent composition in which the bulk density of the detergent composition is increased by spraying detergent particles with a liquid and then dusting with a fine powder in a rotating drum or mixer. It is preferred that the liquid comprise a nonionic surfactant. Optionally, the liquid may also include other ingredients, such as perfume or a slurry, in water, of an optical brightener. The Examples describe the use of a 20% aqueous solution of optical brightener.
However, a problem with mixing a nonionic surfactant with water is that this can lead to the formation of flakes of nonionic surfactant which tend to block the spray nozzle, thereby interrupting the process and requiring cleaning of the nozzle before the process can re-start. Furthermore, the use of an aqueous spray tends to be detrimental to the stability of the final detergent composition because water tends to react with and/or promote reaction of components of the composition.
JP-A-07286198 discloses a process for preparing a granular nonionic detergent composition comprising spray-drying detergent particles containing 1% by weight of a nonionic surfactant and 0.01% by weight of a brightener. The brightener is first dissolved and/or dispersed in the nonionic surfactant, and is then granulated with other ingredients to form detergent particles prior to spray-drying. The process is designed to prevent unevenness of fluorescence of textile articles during washing.